3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB in UMTS, and as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In UMTS, a Radio Network Controller (RNC) controls the NodeB, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. The RNC is also connected to the Core Network (CN). In LTE, the eNodeB manages the radio resources in the cells, and is directly connected to the CN. The eNodeB is also connected to neighboring eNodeBs via an X2 interface.
In LTE a new UE power class (probably 31 dBm) for public safety operation is being standardized. Similarly Uplink Multiple Input Multiple Output (UL MIMO) capabilities in the UE are also being standardized. These capabilities can be exploited to improve the data rate and uplink coverage.
Duplex Modes of Operations
A frequency band or an operating frequency band supports a specific duplex mode of operation. The possible duplex modes are: frequency division duplex (FDD), time division duplex (TDD) and half duplex FDD (HD-FDD).
In FDD mode of operation, which is used in UTRAN FDD and E-UTRAN FDD, the uplink and downlink transmission take place on different carrier frequency channels. Therefore, in FDD mode both uplink and downlink transmission can occur simultaneously in time. On the other hand in TDD mode, which is used in UTRAN TDD and E-UTRAN TDD, the uplink and downlink transmission take place on the same carrier frequency channel but in different time slots or sub-frames. HD-FDD, which is used in Global System for Mobile communications (GSM), can be regarded as a hybrid scheme where the uplink and downlink are transmitted on different carrier frequencies and are also transmitted on different time slots. This means that uplink and downlink transmission don't occur simultaneously in time.
UE Requirements
The UE has to meet a certain set of requirements, which can be classified as transmitter requirements and receiver requirements. The objective of the requirements is to ensure certain performance level on the own UE as well as to not degrade the performance of surrounding UEs or radio nodes such as base stations (BS). Certain radio related requirements are in particular required to be met due to regulation in a country or a region. The requirements are therefore typically pre-defined in the standard.
Examples of transmitter requirements are output power or out of band emission requirements. Out of band emission requirements are more thoroughly described hereinafter. Examples of receiver requirements are receiver sensitivity, Adjacent Channel Selectivity (ACS) or blocking requirements.
Similar requirements are also specified for radio nodes e.g. base stations, relays, and repeaters. However the performance figures for the UE and the radio node are generally different.
Factors Limiting UE Maximum Output Power
The UE maximum output power is limited due to the following major factors:                Radio emissions, e.g. out of band emission and RF exposure        Terminal battery life        Heat dissipation        
These concepts are described below:
Radio Emissions
Although a wireless device typically operates in a well defined portion of a frequency band, emissions outside its channel bandwidth and also outside its operating frequency band are unavoidable. These emissions outside the bandwidth or frequency band are often termed as out of band emissions or unwanted emissions. The emissions both inside and outside the bandwidth and/or frequency band of operation are also exposed to human body.
These two concepts, i.e. unwanted emissions and RF exposure to human, and their associated signaling aspects are described below.
Unwanted Emissions
The UEs as well as base stations have to fulfill a specified set of unwanted emission requirements, which consist of out of band (OOB) emissions and spurious emissions. The objective of OOB emission requirements is to limit the emissions from the transmitters (UE or BS) on frequencies adjacent to their respective channel bandwidths due to for example non-linearity and component imperfections. In fact, all wireless communication standards, such as GSM, UTRAN, E-UTRAN, and Wireless Local Area Network (WLAN), clearly specify the OOB emission requirements to limit or at least minimize the unwanted emissions. Spurious emissions requirements are defined in order to limit the emissions out of the operating band where the UE or BS is operating due to for example harmonic emissions and intermodulation products. The unwanted emission requirements are primarily approved and set by the national and international regulatory bodies. The major OOB and spurious emission requirements are typically specified by the standard bodies and eventually enforced by the regulators in different countries and regions for both UE and BS.
The OOB emissions comprise:                Adjacent Channel Leakage Ratio (ACLR)        Spectrum Emission Mask (SEM)        
The specific definition and the specified level of OOB and spurious emissions can vary from one system to another. Typically these requirements ensure that the emission levels outside the transmitter channel bandwidth or operating band remain several tens of dB below the transmitted signal. Emission levels tend to decay dramatically further away from an operating band but they are not completely eliminated at least in the adjacent carrier frequencies.
Concept of Maximum Power Reduction (MPR)
As stated above that the UE and BS have to meet the OOB and spurious emission requirements irrespective of their transmission power level. For the UE the conservation of its battery power is very critical. This requires that the UE has an efficient power amplifier (PA). The PA is therefore typically designed for certain typical operating points or configurations or set of parameter settings e.g. modulation type, number of active physical channels such as resource blocks in E-UTRA or number of Code Division Multiple Access (CDMA) channelization codes code or spreading factor in UTRA. But in practice the UE may operate using any combination of modulation, and physical channels. Therefore, in some uplink transmission scenarios the UE power amplifier may not be able to operate in the linear zone, thereby causing unwanted emissions due to harmonics or other non-linear characteristics. To ensure that UE fulfills OOB/spurious requirements for all allowed uplink transmission configurations the UE is allowed to reduce its maximum uplink transmission power in some scenarios. This is called MPR or UE power back-off in some literature. For instance a UE with maximum transmit power of 24 dBm power class may reduce its maximum power from 24 dBm to 23 or 22 dBm depending upon the configuration. It should be noted that the term maximum transmit power may interchangeably be referred to as maximum transmission power or maximum output power. All these terms have the same meaning.
The BS may also have to perform MPR. However, this is not standardized. Secondly the BS can afford to have a PA with larger operating range since its efficiency is less critical compared to that of UE. The MPR values for different configurations are generally well specified in the standard. The UE uses these values to apply MPR when the conditions for the corresponding configurations are fulfilled. These MPR values are regarded as static in a sense that they are independent of resource block allocation and other deployment aspects.
Concept of Additional MPR, A-MPR
In E-UTRA an Additional MPR (A-MPR) for the UE transmitter has also been specified in addition to the normal MPR. The difference is that the former is not fully static. Instead the A-MPR can vary between different cells, operating frequency bands and more specifically between cells deployed in different location areas or regions. In particular the A-MPR may be applied by the UE in order to meet the additional emission requirements imposed by the regional regulatory organization. A-MPR is an optional feature, which is used by the network when needed depending upon the co-existence scenario.
The A-MPR defines the UE MPR, on top of the normal MPR, needed to fulfill certain emission requirements by accounting for factors such as: bandwidth, frequency band or resource block allocation.
In the following, signaling of regulatory requirements and A-MPR to the UE is described. The regulatory requirements may vary from one region to another and from one network to another. The presence of additional regulatory requirements is signalled via a cell specific signalling known as network signalling (NS). Associated with the NS signalling there is a set of A-MPR values which may depend on for example resource block allocation, channel bandwidth, frequency band, or non-cellular systems (e.g. public safety wireless network).
To meet the regulatory emission requirements the A-MPR required could vary from one part of the network to another. This is due to the factors such as the variable bandwidth, varying number of resource block allocation, and different bands in different parts of the networks. Even if the deployment scenario in terms of e.g. bands used, and bandwidth size is homogeneous in a large coverage area, there will always be border regions between these coverage areas. Therefore A-MPR is a cell specific value.
Due to the above reasons the NS value is signaled to the UE via system information in a UE specific channel or in a broadcast message. This allows the UE to acquire this information when it camps on to a cell. The acquired NS value which is associated with a cell is then used by the UE to map to certain A-MPR and reduce its maximum output power whenever it transmits in the uplink.
RF Exposure to Human
Another important factor is the human exposure to Radio Frequency (RF) Electromagnetic Fields (EMF) which are transmitted by the UE. The most important guidelines on RF exposure to human are from the International Commission on Non-Ionizing Radiation Protection (ICNIRP, 1998) and from the Institute of Electrical and Electronics Engineers (IEEE, 1999). The limits in these recommendations are similar and they have been used as the basis for national standards and regulations in many countries. The ICNIRP guidelines, which are the most widely used recommendations, have been endorsed by the World Health Organization.
These RF exposure guidelines are science-based and the prescribed limits have been set with substantial safety margins. They provide protection from all established health effects from short-term and long-term exposure to RF fields, and the safety of children and other segments of the population have been taken into account.
Specific Absorption Rate (SAR) is introduced to measure impact on the human body from the exposure of RF EMF transmitted by the UE. SAR is a measure of the maximum energy absorbed by a unit of mass of exposed tissue of a person using a mobile phone, over a given time or more simply the power absorbed per unit mass. Advised by ICNIRP, the communication administration departments of different countries issued the SAR limits. For instance, the Federal Communications Commission (FCC) has determined that the SAR limit is 1.6 W/kg for cell phone. The SAR limit in Europe and in most of countries is 2 W/kg.
Power Reduction to Limit RF Exposure
The UE should comply with the SAR requirements or any type of requirements for limiting the RF exposure to human which are specified by the regulator in an individual country, region, province or state. In order to meet these requirements the UE may also have to reduce its maximum output power. Hence the UE maximum output power is limited by the SAR limit.
In prior art a generic term called power management is also interchangeably used for controlling emissions to limit the SAR. The power management MPR (P-MPR) is the amount of UE output power reduction needed to meet the RF exposure requirements.
The following describes the signaling of RF Exposure Requirements to the UE. In prior art one or more parameters associated with the MPR to be applied by the UE to meet the SAR or any type of RF exposure requirements are signaled to the UE. This means the P-MPR may also be signaled to the UE. This is due to the fact that SAR or RF exposure requirements may vary from one region to another. Hence the amount of the MPR required by the UE to meet the requirements may vary from one cell to another.
Terminal Battery Life
If the maximum output of the UE transmitter is high then the battery life will be affected. Therefore it is desirable that the UE operates at higher output power only when necessary.
Heat Dissipation
Another important factor in limiting the UE output power is the impact on the heat dissipation. When the UE operates at higher output power the temperature of the components and circuitry typically increases. The increase in the temperature may lower the accuracy with which the UE can perform various physical layer operations e.g. power control, emission control, and radio measurements. Hence it is not desirable that the UE often operates at a higher output power than necessary.
Public Safety Operation
Public safety wireless communication system relates to the prevention of and protection from dangerous and hazardous events, e.g. a natural or man-made disaster such as tsunami, earthquake, flood, fire, or an act of terror.
The existing Public safety wireless system is typically a narrowband system, from 6.25 to 25 kHz bandwidth, depending on the part of the allocated spectrum. However, the use of a wideband or broadband technology for Public Safety is now getting a lot of interest. In wideband or wide bandwidth technology larger bandwidth such as 5 MHz or 10 MHz may be used. The wideband technology will enhance the type and quality of communication. For example it will allow the public safety personnel to send emergency signal and respond to them in a faster and more efficient way than today. It will also provide the public with new ways of calling for help and receiving help, e.g. receiving a video with instructions regarding what to do in the wake of disaster or in an anticipation of an impending disaster.
Examples of Higher UE Output Power Class
In High Speed Packet Access (HSPA) the most commonly used UE power class is 24 dBm i.e. UE maximum output power, also known as nominal maximum output power, is 24 dBm. In LTE as of now the UE power class is 23 dBm. However there are features which may require UE to support higher output power class. A few examples are given below.
High UE Power Class for Public Safety
Recently a work item to specify UE requirements for E-UTRA UE Power Class 1 (+31 dBm maximum output power or more) has been approved. More specifically this higher power class is being specified to operate in the Public Safety broadband (PSBB) deployment in band 14 (700 MHz) in US. The PSBB will use LTE technology. This means their maximum output power is well above the normal maximum output power level used for mobile/cellular communication, which is e.g. 23 dBm in LTE as described above. The normal maximum output power herein refers to the level which is within the SAR limit. As described earlier this SAR limit ensures protection from health effects due to exposure to RF fields.
The Unused UE Potential Power in Case of Uplink Multiple Antenna Transmission
Taking HSPA evolution as an example, the Uplink Transmit Diversity (ULTD) is being specified in 3GPP for HSPA. FIG. 1a shows the UE structure of Open Loop Beam-Forming (OLBF). A typical OLBF implementation comprises two transmit antennas, 15a, 15b, using two half PAs, 11a, 11b, (e.g. each of 21 dBm). In such implementation, the Random Access Channel (RACH) performance may not be guaranteed in case the beamforming is also used for the RACH transmission, since no feedback information is available to generate an accurate RACH beam. Alternatively the use of a single PA may cause loss in RACH coverage especially when UE is located in a cell border region. In order to ensure good coverage of the RACH access, an alternative implementation was proposed where one of the two PAs, 11a, 11b, could be full-power PA. The full power PA could for instance be used for RACH without beamforming to ensure the coverage equivalent to that of the legacy RACH. The full-power PA can output up to the maximum allowed output power of the UE i.e. the nominal maximum output power (e.g. 24 dBm). This means the OLBF capable UE with this alternative PA implementation can output a higher power than the maximum allowed transmit power when two PAs output the maximum power at the same time. Hence the UE potential power cannot be fully utilized due to the limitation of maximum allowed UE output power. In other words the UE cannot transmit up to its actual output power capability. In case both PA1, 11 a, and PA2, 11 b, are full-power amplifiers, 3 dB of the UE total potential power remains unused. In case PA1 is a full-power PA and PA2 is a half-power PA, 1.76 dB of the UE total potential power remains unused.
One example of a UE architecture supporting Closed Loop Transmit Diversity (CLTD), i.e. baseband processing and radio frequency (RF) front end, is illustrated in FIG. 1b. The UL MIMO is another potential natural step in case CLTD is used since there are no major differences in their architectures. In order to ensure legacy RACH coverage, the UE capable of CLTD and UL MIMO may also employ one or more full power PA. The UE with such implementation cannot operate with its full output power capability leading to power wastage.
Carrier Aggregation
To enhance peak-rates within a technology, multi-carrier or carrier aggregation solutions are known. For example, it is possible to use multiple 5 MHz carriers in HSPA to enhance the peak-rate within the HSPA network. Similarly in LTE for example multiple 20 MHz carriers can be aggregated in the uplink and/or in the downlink. Each carrier in multi-carrier or carrier aggregation system is generally termed as a component carrier (CC) or sometimes is also referred to as a cell. In simple words the CC means an individual carrier in a multi-carrier system. The term carrier aggregation (CA) is also called “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the primary carrier or anchor carrier and the remaining ones are called secondary or supplementary carriers. Generally the primary or anchor CC carries the essential UE specific signaling. The primary CC exists in both uplink and downlink direction. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The CCs belonging to the CA may belong to the same frequency band, also known as intra-band CA, or to different frequency bands, inter-band CA, or any combination thereof, such as e.g. 2 CCs in band A and 1 CC in band B. The inter-band CA comprising carriers distributed over two bands is also called dual-band-dual-carrier-High Speed Downlink Packet Access (HSDPA) in HSPA or inter-band CA in LTE. Furthermore the CCs in intra-band CA may be adjacent or non-adjacent in frequency domain, the latter also known as intra-band non-adjacent CA. A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using CA between carriers of different technologies is also referred to as “multi-Radio Access Technology (RAT) carrier aggregation” or “multi-RAT-multi-carrier system” or simply “inter-RAT carrier aggregation”. For example, the carriers from WCDMA and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity the carrier aggregation within the same technology as described can be regarded as ‘intra-RAT’ or simply ‘single RAT’ carrier aggregation.
The FDD or HD-FDD capable UE supporting carrier aggregation requires RF devices like diplexer or quadplexer, which result in UE maximum output power reduction. The power reduction affects loss of uplink coverage even when such a UE operates in legacy mode, i.e. in single carrier operation.
RACH Coverage
Physical random access channel (PRACH) is used by the UE during a random access procedure. In HSPA the PRACH is used by the UE for accessing the cell during initial access e.g. in idle mode, during RRC re-establishment, or RRC connection release with redirection. In LTE the PRACH is used by the UE for accessing the cell during initial access, e.g. in idle mode, during RRC re-establishment, or RRC connection release with redirection, but also during handover when accessing a neighbor cell.
The UE sends a selected predetermined sequence over PRACH when it wants to setup a radio connection or perform a cell change, e.g. in LTE. The network should detect the PRACH sequence and send a response to the UE to continue the radio link establishment. In order to reduce the delay in radio link setup, the time between the UE sending the PRACH sequence and the network sending the PRACH response is very short, e.g., less than 2 ms for WCDMA-HSPA. The UE typically transmits the PRACH with an output power level which is determined based on an open loop power control principle. This means that the UE first estimate the uplink path loss and determine the power to achieve certain target signal level at the base station. The target signal level or associated parameters are signaled to the UE by the relevant network node.
Advanced Receivers
A well known example of an advanced receiver is a Minimum Mean Square Error Interference Rejection Combining (MMSE-IRC). Examples of more sophisticated advanced receiver are Mean Square Error-turbo Interference Cancellation, and post-decoding successive interference cancellation receiver. They are capable of performing nonlinear subtractive-type interference cancellation. This can be used to further enhance system performance.
The terms interference mitigation receiver, interference cancellation receiver, interference rejection receiver, interference aware receiver, interference avoidance receiver etc are interchangeably used but they all belong to a category of an advanced receiver or an enhanced receiver. All these different types of advanced receiver improve performance by fully or partly eliminating the interference arising from at least one interfering source.
The advanced receiver or an enhanced receiver can be used in the UE or in a radio network node, e.g. BS. The base station may use the advanced receiver for receiving different types of channels including PRACH.
Problems with Existing Solutions
There exist UEs with higher output power class developed for specific operation and scenarios e.g. for public safety. That is their nominal maximum output power is higher than the normal level used for mobile communication. However in prior art there are no mechanisms or procedures for how to use such UEs for another set of scenarios, such as for normal cellular operation, while meeting the normal requirements i.e. requirements related to cellular operation. Hence such a higher power class UE has to meet the same level of requirements, i.e. suitable for a particular type of network operation, regardless of the criticality of the network operation. This may not be optimal from a network operation perspective in some scenarios and may also prevent mobility between different systems, e.g. between public safety network and normal mobile communication networks. An operator may e.g. not allow a UE with higher maximum output power to operate as a cellular UE.
On the other hand the uplink coverage in many scenarios is becoming a severe bottleneck. For example the UE power back-off is introduced due to losses when signal passes through RF components like diplexer or quadplexer. These components are particularly used for multi-band and multi-carrier transmission and reception, multi-antenna transmission, and combinations thereof. The UE power back-off in such implementation is also required when the UE operates in legacy mode e.g. single band operation or single antenna operation. This means that a high-end UE, such as a UE adapted for multi-carrier transmission, may have worse coverage and performance when operating in legacy mode compared to a low-end UE e.g. adapted for single carrier transmission. This can be annoying for the end user and for the operators. Furthermore in certain critical scenarios and situations it is important that the UE is able to maintain coverage and connection with the network.